EXCERPT: More than 10,000 animals will be ferried out of the [California] area this year by Rescue Express, one of the dozens of organizations across the nation fueling a dizzying daily reshuffle of dogs and cats by car, van, bus, and private and even chartered plane…

…This overground pet railroad existed on a small scale for years, then rapidly expanded in the eastern United States after Hurricane Katrina left thousands of animals homeless in 2005. Transports more recently have mushroomed in the West, despite concerns in some places about what remains a fairly unregulated practice…

These transports, mostly from high-kill southern regions, are small but growing factors in a long-term decline in euthanasia at U.S. shelters. According to some estimates, animal shelters killed as many as 20 million cats and dogs annually in the 1970s. That had fallen to 2.6 million by 2011 and to 1.5 million today, according to the American Society for the Prevention of Cruelty to Animals.

The numbers are only approximations, because no central data collection exists and only some states require shelters to report intake and outcome figures. But animal advocates agree that the decrease in euthanasia has been dramatic, driven mostly by successful spay-neuter programs and, more recently, by savvy adoption campaigns, greater efforts to reunite lost pets with owners and the proliferation of advocacy groups both small and large that have swept in to help municipal shelters, often poorly funded and sluggish…

…shelters and rescue groups say an increasing number of communities in northern parts of the country now take in migrants — young and old, small and large. Nearly a third of the 30,000 dogs and cats received by a Portland, Ore., coalition of six shelters in 2016 came from outside the area, including from Hawaii…